The present invention is directed to the recovery of waste heat from chemical reactions. More particularly, the invention relates to a waste heat boiler with improved control of cooling effect.
Waste heat boilers are most generally used for the generation of steam by waste heat recovered from hot process streams. Typically, those boilers are designed as shell-and-tube exchangers with a plurality of heat exchanging tubes arranged within a cylindrical shell.
Two basic types of shell-and-tube exchangers are employed in the industry, the watertube type, in which water/steam mixtures flow through the tubes, and the fire tube type having the heating process stream inside the tubes.
The characteristic components of the boiler are the tubes mounted in tubesheets at a front-end head and a rear-end head within the shell. In the firetube boilers steam production is accomplished on the shell side of the tubes by indirect heat exchange of a hot process stream flowing through the boiler tubes. The shell side is through a number of risers and downcomers connected to a steam drum, which may be arranged at the top of the boiler shell.
The mechanical design and, in particular, dimensioning of the heat exchanging surface in shell-and-tube exchanger type boilers represent certain problems. Boiler applications involve high pressures on the shell side and considerable temperature differences between the shell-and-tube side. Particular considerations have to be given to fouling and corrosion characteristics of the process stream.
Boilers handling fouling or corrosion prove process streams must be designed to a higher duty than required in order to allow for satisfying lifetime under serious fouling and corroding conditions. The heat transferring surface of the boiler tubes has further to be adapted to expected corrosion and fouling factors in the stream. To provide for a desired and substantially constant cooling effect during long term operation of the boilers, appropriate heat transfer and temperature control is required.
Conventionally designed boilers are equipped with a by-pass of a large diameter tube, which may be internal or external to the boiler shell. The by-pass is usually construed as an insulated tube provided with a flow control valve. During initial operation of the boilers, part of the hot process stream is by-passed the heat transferring tubes to limit the heat transfer within the required level.
After a certain time on stream fouling and corrosion of the tubes increase, leading to decreased heat transfer. The amount of by-passed process stream is then reduced, which allows for higher flow of the process stream through the heat transferring tubes to maintain the required cooling effect.
A major drawback of the known boilers of the above type is vigorous corrosion on the metallic surface of the by-pass and flow control valve, which are in contact with the uncooled process stream at temperatures as high as 1000.degree. C.